But the problem is, although I understand it a little tiny bit, there doesn't seem to be a system dbus reference showing what signals are broadcast by the system (at least I couldn't find any such documentation).
You can use dbus-monitor in your terminal to show the dbus messages that are being sent on the dbus pipes. As it will show all dbus messages, I would suggest that you closes all applications you don't need in order to only get dbus messsages that you're interested in.
dbus-monitor --system